A phase-locked loop circuit (PLL) is used to generate a reference clock signal in various electronic circuits. A PLL has an oscillator that generates an oscillation signal of a frequency corresponding to an external signal. As one of various known oscillators, there is a current-controlled oscillator in which a frequency of an oscillation signal is controlled by an input control current. The control current is generated by, for example, a current source capable of controlling an amount of a current, and a current mirror copying a current flowing through the current source.
However, there is a problem that when the power-supply voltage of the oscillator fluctuates, the control current also fluctuates. As a result, it is difficult to generate a precise oscillation signal.